Set CR_CLANG_REVISION to force a rebuild when the Clang version changes.
Set CR_CLANG_REVISION to force a rebuild when the Clang version changes.

BUG=pdfium:335
R=thakis@chromium.org

Review URL: https://codereview.chromium.org/1549863002 .

DrAlexx/pdfium,andoma/pdfium,DrAlexx/pdfium,DrAlexx/pdfium,andoma/pdfium,andoma/pdfium,DrAlexx/pdfium,andoma/pdfium